1. Field of the Invention
The present invention relates to a pneumatic tire in which a land portion such as a block, a rib and the like is provided in a tread and a sipe is formed on the land portion, and particularly it relates to a pneumatic tire having an improved tire performance on ice and snow.
2. Description of the Related Art
Because of lowering of coefficient of friction .mu. on a frozen road due to mild winters, it is further desired to improve a brake performance on ice of a studless tire.
Generally, a plurality of blocks are formed on a tread of the studless tire and a plurality of sipes are formed on these blocks.
In order to improve a tire performance on ice of the studless tire, the conventional art includes an increase of frictional force by increasing a land contact surface area (a positive surface area) and an improvement of digging up effect by increasing a number (a density) of sipes and increasing an edge component.
Further, on the ice at a temperature close to 0.degree. C., it is known that removing a water film springing out to the road contact surface is important for improving the tire performance on ice.
In order to achieve this, it is effective to increase the density of sipes. However, if the density is increased too much, a volume of the sipe within the land portion is increased due to a thickness of the sipe and a rigidity of the block is lowered so that there is a problem that the road contact surface is decreased.
In order to avoid this problem, it is considered to make the thickness of the sipe thin. However, in accordance with this method, the sipe is closed at a time of receiving a load so that an effect of removing the water film sprung out into the road contact surface is decreased. To mention in detail, as shown in FIGS. 19A and 19B, a block 18 which is not in contact with a road surface has a sipe 20 of a predetermined thickness. When the block 18 is brought into contact with the ice road surface and a load is applied thereto, the block 18 is deformed as shown in FIG. 20B so that the sipe 20 is closed as shown in FIGS. 20A and 20B. Accordingly, an effect of removing a water (a water film) 26 generated on an ice road surface 24 is decreased.